1. Field of the Invention
This invention relates to a magnetoresistive reproduce head, and in particular to a head of simplified structure and improved operating characteristics.
2. Description Relative to the Prior Art
The magnetoresistive (MR) reproduce head is well known in the art; its high signal output and ease of manufacture by means of batch fabrication techniques have made it an attractive candidate for use in modern digital data devices. The unshielded MR head, disclosed in U.S. Pat. No. 3,493,694 issued in the name of R. P. Hunt, has been improved and elaborated upon, and U.S. Pat. Nos. 3,840,898, 3,864,751, and 3,940,797 reflect such improvements and elaborations. Included among the improvements are a variety of hard axis biasing techniques for linearizing the signal response of the MR element whose inherent resistance change as a function of magnetic field is quadratic in nature.
The function of the hard axis biasing of the MR element may be understood by referring to FIG. 1. The ordinate of the curve 10 depicts the percent change of resistance of the MR element as a function of the applied magnetic field H plotted along the abscissa. It will be observed that the shape of the curve 10 is substantially non-linear over the major portion of its range. For the d.c. hard axis biasing case, which is the conventional hard axis biasing method for an MR element, the bias point 12 is selected in the most nearly linear portion of the curve 10, and this is essentially at the point of inflection 14 of the curve 10. The value of the field H is then established at the value Hb to set the hard axis bias at the point of inflection 14, and the signal to be reproduced, Hs, varies the magnetic field about the bias value Hb with a resulting detectable change in the MR element's resistance 18.
The principle of biasing by means of a soft adjacent layer (SAL) may be understood by referring to FIG. 2. A magnetoresistive layer 20 is adjacent to, and insulated from, an SAL biasing layer 22. The MR layer may be 400 angstroms thick, the SAL layer 300 angstroms thick, each layer being 5 microns high and approximately as wide as the magnetic track width. The distance between the MR and SAL layers may be on the order of 1000-3000 angstroms. In the art, the MR and SAL elements have generally been fabricated from the same or similar materials. For example, the MR element may be made of a Ni-Fe alloy, e.g. permalloy, while an SAL layer may be fabricated from Ni-Fe alloy or other alloys such as CoZr alloys. In the conventional operation of the MR head, a d.c. sense current 24, which serves a dual purpose, flows in the MR element 20. The resistance of the MR element 20 is modulated by the signal field of the medium, and the current 24 flowing through the modulated magnetoresistance provides a baseband signal voltage which is the head output. Because of the presence of the SAL 22, the current 24 is also the origin of the hard axis bias field at the MR element 20. A magnetic field Hi due to the current 24 in the MR element 20 magnetizes the SAL 22 which, in turn, generates the hard axis bias field Hb which rotates the MR element's magnetization vector 26, establishing the hard axis bias point of the MR element 20. The prior art teaches operation of the SAL in a magnetically saturated state to keep the hard axis bias field magnitude, Hb, independent of any variations in Hi resulting from variations in the sense current 24.
Referring to FIG. 3, an MR head of the prior art is deposited on a substrate 30, which is a wear resistant, non-magnetic substance such as a silicon or AC2, i.e. (Al.sub.2 O.sub.3 +TiC), wafer. An insulating layer 32, such as SiO.sub.2 is then laid down, followed by a deposited SAL element, 34. Another insulating layer 36 is then deposited to a thickness of from 1000-3000 angstroms followed by the deposition of the MR element 38. The final deposition step lays down another insulating layer 40, and then a cover plate of wear resistant material 42 is secured to the head structure completing the assembly. The direction of tape travel is indicated by an arrow 44, and the head is contoured by a lapping process to provide a contour 46 whereby the tape is in intimate contact with the head contour 46 during operation. Recalling that the separation between the SAL element 34 and the MR element 38 is only on the order of 1000-3000 angstroms, and that the lapping process selectively abrades the surface of the head to provide the contour 46, a problem in the prior art has been the "smearing" of the mechanically soft SAL and MR elements during lapping with resultant electrical "shorts" between these elements. The present invention circumvents this problem by means of a hard axis biasing technique that eliminates the need for a mechanically soft deposited SAL element located in proximity to the MR element within the narrow gap of the head.